Is it worth changing gears for better MPG's???

[TurboDiverArt]

Art, The GM-8 exhaust turbine "exhaust outlet" is some of the limiting factor needing some porting. How much porting? It's more a matter of how much risk one is willing to take making walls too thin.

Some say the elbow after the gate is an issue, then some smoothing out of its inside surfaces are in order.

I really thing wee need those bigger Holset 35, 40 turbo's providing we stay out of the surge line, and keep boost at a level of 15+- psi @ intake.

If the GM-8 seems to run out of steam at 2800, were does a GM-8 run out with a free flowing exhaust? Mine definitely seems to lose it by 3100, maybe 3000.

Art.

[ronniejoe]

The rotors on a GM-4 and GM-8 are identical. The only difference is a slightly better compressor housing and a slightly better exhaust elbow. The waste gate was moved out of the exhaust housing into the elbow on the 8's and I've been told that the elbow is more efficient. With that said, I don't think you'll see significant performance differences between the two. The wastegate move was to improve reliability of the bushing (mine failed) and had nothing to do with performance.

I have an outline map of a the GM series compressors...it has no detail and IHI will not even respond to my inquiries. The other manufacturers have been very responsive to my questions.

It is fair to take my comments as being applied to the GM-X series in most cases.

My main comment here was to clarify the relationship between peak torque and peak power and to give you some of the rationale behind choosing shift points. The turbo talk was simply to help explain some of your observations.

[TurboDiverArt]

The rotors on a GM-4 and GM-8 are identical. The only difference is a slightly better compressor housing and a slightly better exhaust elbow. The waste gate was moved out of the exhaust housing into the elbow on the 8's and I've been told that the elbow is more efficient. With that said, I don't think you'll see significant performance differences between the two. The wastegate move was to improve reliability of the bushing (mine failed) and had nothing to do with performance.

I have an outline map of a the GM series compressors...it has no detail and IHI will not even respond to my inquiries. The other manufacturers have been very responsive to my questions.

It is fair to take my comments as being applied to the GM-X series in most cases.

My main comment here was to clarify the relationship between peak torque and peak power and to give you some of the rationale behind choosing shift points. The turbo talk was simply to help explain some of your observations.

Thanks for the info, it's good information! It definitely seems that the stock turbo putters out in the 3000 range. This is why I thought the shift point should be at about that point. It just seems to take forever to get from 3000 to 3500. Now if you had a turbo that would pull all the way up to 3500 RPM

[ronniejoe]

[quote=TurboDiverArt]Now if you had a turbo that would pull all the way up to 3500 RPM

[john8662]

Hold that thought...

You just couldn't resist could you!